It is generally held by Christians that angels sinned before man though some have held that the fall of the angelic realm did not happen until the devil tempted Eve which would then make it simultaneous.  But, this doesn’t seem to be the case.  Nevertheless, Adam had not yet fallen in the Garden of Eden when the devil was tempting Eve. Therefore, it would seem logical to say that Satan had fallen well before his entrance into the Garden of Eden.  Let’s take a look at two important sets of verses that are commonly believed to be about the Devil.

• Ezekiel 28:13-15, “You were in Eden, the garden of God. Every precious stone was your covering: The ruby, the topaz, and the diamond; The beryl, the onyx, and the jasper; The lapis lazuli, the turquoise, and the emerald; And the gold, the workmanship of your settings and sockets, was in you. On the day that you were created they were prepared. 14 You were the anointed cherub who covers, and I placed you there. You were on the holy mountain of God.  You walked in the midst of the stones of fire. 15 You were blameless in your ways from the day you were created, until unrighteousness was found in you.”
• Isaiah 14:12-15, “How you have fallen from heaven, O star of the morning, son of the dawn! You have been cut down to the earth, You who have weakened the nations! 13 But you said in your heart, ‘I will ascend to heaven; I will raise my throne above the stars of God, and I will sit on the mount of assembly in the recesses of the north. 14 ‘I will ascend above the heights of the clouds. I will make myself like the Most High.’ 15 Nevertheless you will be thrust down to Sheol, to the recesses of the pit.”

The above scriptures teach us that Lucifer was exceedingly beautiful and that he lifted himself up in pride and wanted to be like God. In this, he sinned (along with a third of the angels, Revelation 12:4) and then he eventually entered the Garden of Eden and tempted Eve.

So, it seems obvious that the angelic realm sinned before Adam did.